Strongly Correlated Electrons
Machine learning magnetic interactions from neutron powder diffraction data
Adit S. Desai, Yongqiang Cheng and Joseph A. M. Paddison
Neutron diffraction is a versatile experimental technique capable of probing a material's
magnetic properties. While diffraction is typically used to determine the
magnetic structure of a material,
magnetic diffuse scattering data from a diffraction experiment are also sensitive to the
magnetic interactions in its
Hamiltonian. However, accurately determining
magnetic interaction parameters from
neutron-scattering data involves an inverse scattering problem that is challenging to solve in general. Here, we investigate the effectiveness of a machine learning approach to predict the interaction parameters given
magnetic diffuse-scattering data measured on powder samples, for a comprehensive survey of isotropic interactions on eight high-
symmetry lattices. Across all
lattices we considered, the machine-learning approach estimates the interaction parameters with high (~2%) accuracy, while avoiding the issue of false minima that is encountered with non-linear least squares refinement. Our results highlight that powder diffuse-scattering data can provide a compact "fingerprint" of the
magnetic interactions for many materials.
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Soft Condensed Matter
Active colloidal rafts perform swimming strokes that propel their host droplets
Airi N. Kato, Julien Brémont, Etienne Harté et al. (+3 more)
Active particles confined in droplets can set their host droplet into motion. This phenomenon is usually attributed to direct hydrodynamic forcing by the enclosed particles. Here we reveal a different mechanism: we show that a self-propelled raft of active colloids confined in a thin oil droplet at the surface of water typically follows an off-centered orbit, thus periodically deforming the droplet's contact line in an anisotropic way. This non-reciprocal deformation acts as a swimming stroke and propels the droplet according to the principles of low-Reynolds-number swimming. The effect is absent for a single colloid and arises from the intrinsically many-body dynamics of the raft, which we capture with a dynamical model coupling viscous hydrodynamics and many-body active dynamics. Our results show how collective internal activity can be converted into locomotion through deformation of a soft confining boundary.
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All authors Airi N. Kato, Julien Brémont, Etienne Harté, Jean-Michel Rampnoux, Jean-François Joanny, Hamid Kellay
Mesoscale and Nanoscale Physics · Cross-listed from physics.optics
Quantized Transport of Gap Solitons in the Harper-Hofstadter Model
Oliver Ashfield, Henry Davenport, Tom Sheppard et al. (+6 more)
We investigate the dynamics of gap
solitons found in the Harper-Hofstadter model with an additional cubic nonlinearity, such as that describing weakly-interacting cold atoms or a Kerr nonlinearity in an optical platform. We find that certain
solitons with frequencies deep in the band gap travel with a robust, quantized velocity perpendicular to the applied force and maintain their spatial profile over long timescales. Crucially, this velocity is linearly proportional to the applied force, allowing for arbitrary steering of the nonlinear wavepacket, without breakdown, using only a tunable external force. This quantized transport persists despite a highly non-uniform occupation of the linear Bloch states, and contrasts with
solitons near the band edge, which deform and delocalize under the same force. These results point to a robust and highly controllable mechanism for steering nonlinear
solitons in experimentally accessible platforms.
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All authors Oliver Ashfield, Henry Davenport, Tom Sheppard, David G. Reid, Holly A. J. Middleton-Spencer, A. C. Berceanu, Frank Schindler, Iacopo Carusotto, Hannah M. Price
Materials Science
Low resistance NiO/\beta-Ga_{2}O_{3} heterojunction diodes grown via molecular beam epitaxy
Dagny Sacksteder, Anna Sacchi, Renae Gannon et al. (+4 more)
NiO is one of the most important p-type oxide contact materials used in many
semiconductor technologies. However, current NiO growth methods can induce interfacial damage that diminishes device performance. Fine control of interfaces is especially important in implementing NiO heterojunction diodes and transistors based on ultra wide band gap (UWBG)
semiconductors such as AlGaN and Ga_{2}O_{3} used for power electronic applications. Here, we report on how molecular beam epitaxy can be used to achieve low-defect, lightly doped NiO contact layers for a \beta-Ga_{2}O_{3} diodes. Although high-temperature growth does not measurably decrease the on-state resistance of the diode, increased growth rates up to 600 nm/hr lower on-state resistance in p– NiO / \beta-Ga_{2}O_{3} heterojunction diodes without reducing film quality. At a NiO growth rate of 380 nm/hr, unoptimized diodes with 35 nm thick p– NiO layers demonstrate a device-average specific on-state resistance of 1.46 \Omega-cm^{2} and an ideality factor of 1.46. Individual devices grown at this condition show specific on-state resistance as low as 25 m{\Omega}-cm2 with a rectification ratio of 2.7x106. Scanning transmission electron microscopy imaging reveals the (100) NiO/ (100) \beta-Ga_{2}O_{3} interface is coherent and atomically abrupt. These results open a new avenue to optimizing the NiO interface to produce robust, competitive kV-class power electronic devices based on \beta-Ga_{2}O_{3} and other UWBG
semiconductors.
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All authors Dagny Sacksteder, Anna Sacchi, Renae Gannon, Michelle Smeaton, Megan E. Holtz, Andriy Zakutayev, M. Brooks Tellekamp
Statistical Mechanics
Multi-Resolution Wire-Fencing for Efficient Path Sampling
Simen Z. Stenersen Michler, Lukas Baldauf and Titus S. van Erp
Path sampling methods enable the computation of thermodynamic and kinetic properties through Monte Carlo (MC) moves that generate trajectories from short forward and backward molecular dynamics (MD) segments. Recently, the wire-fencing move was developed to achieve near-unity acceptance while also rapidly decorrelating successive paths, two properties that are usually in conflict in conventional MC schemes. However, in large systems, such as biomolecular simulations, the frame-saving frequency is often kept low to reduce storage requirements. Likewise, when evaluating the order parameter is expensive, frames are saved less frequently to reduce the associated cost. In either situation, this can severely limit the number of available shooting points, in extreme situations leaving only a single point, typically the one with the highest order parameter value, accessible for shooting. Repeated shootings may then originate from the same configuration, reducing sampling efficiency. Here, we introduce a multi-resolution variant of the wire-fencing move in which selected subtrajectories are propagated at higher temporal resolution than the stored trajectories. This refinement affects only the MC move and does not alter the structure or storage of the generated paths, but enhances the
diffusion of shooting points along the trajectory and thereby improves sampling efficiency. The approach is demonstrated on two model systems and a realistic
protein–ligand unbinding process, with the latter showing an estimated efficiency improvement of more than an order of magnitude.
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Materials Science
PhonoMC: Occupation-based deviational Monte Carlo for phonon transport with temperature-dependent scattering
Shixian Liu, Fei Yin, Gang Wang et al. (+4 more)
Nanoscale self-heating involves
phonon transport across confined geometries, material interfaces, and temperature fields over which the scattering rates can vary substantially. We develop PhonoMC, an occupation-based deviational Monte Carlo method for solving the
phonon Boltzmann transport equation within the relaxation-time approximation. A fixed
equilibrium state is retained as the deviational reference, while the local temperature reconstructed from the represented energy is used to evaluate mode-dependent scattering rates. Collisions are updated with a separately determined relaxation temperature to conserve energy over each time step, and prescribed
lattice heating is introduced by changing carrier occupations rather than continuously adding computational particles. For cross-plane transport through a 100-nm Si film, the deviational formulation reproduces the full-population heat flux while reducing its standard deviation by a factor of approximately 3.3 at \(\Delta T=100\)~K with \(10^5\) carriers. In contrast, keeping the scattering rates fixed at 300~K overestimates the heat flux by 23.7% at \(\Delta T=250\)~K. Calculations of Si thin films distinguish finite-length effects from surface-scattering suppression, and Si/3C-SiC bilayers are used to examine interfacial thermal resistance. The method is further applied to localized heating in FinFET-like structures, where replacing the lower Si substrate with higher-conductivity 3C-SiC leads to a higher hotspot temperature because of the additional resistance associated with the confined Si region and the Si/SiC interface. These results show that a fixed deviational reference can be combined with local temperature-dependent scattering and sustained heat deposition in a mode-resolved Monte Carlo description of nanoscale thermal transport.
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All authors Shixian Liu, Fei Yin, Gang Wang, Bin Liu, Ge Zhang, Alexander A. Barinov, Ke Xu
Superconductivity
Tunneling Spectroscopy Study of Low Baked Bulk Niobium for Superconducting RF Cavity Applications
Ivana Curci, Gregoire Jullien, Luc Maurice et al. (+3 more)
Point-contact tunneling (PCT) spectroscopy was used to probe the local superconducting properties of cavity-grade niobium following surface treatments representative of superconducting
radio-frequency (SRF) cavities. Electropolished (EP) samples were compared with samples baked at 120~$^\circ$C for 48 h and subjected to a two-step bake at 75~$^\circ$C for 2 h followed by 120~$^\circ$C for 48 h. Although the superconducting gap $\Delta$ is predominantly distributed around the bulk Nb value (~1.5 meV), regions exhibiting strongly suppressed gaps (minigaps) are observed in 15–29% of junctions on EP samples. Their occurrence decreases to 6% after the 120~$^\circ$C bake and to only 1% after the two-step treatment. Spectra exhibiting minigaps are well described by the Usadel proximity-effect model, consistent with superconducting Nb coupled to normal regions that we attribute to hydride formation. These results provide direct microscopic evidence that low-temperature baking suppresses regions of degraded surface superconductivity in cavity-grade Nb. The pronounced reduction achieved by the two-step bake further supports a link between hydride-related proximity effects and the surface superconducting properties governing SRF cavity performance.
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All authors Ivana Curci, Gregoire Jullien, Luc Maurice, Fabien Eozenou, Patrick Sahuquet, Thomas Proslier
Superconductivity
Strong-coupling multigap superconductivity in the Heusler compound ScAu$_2$Al
Jozef Kačmarčík, Zuzana Pribulová, Yevhen Petrenko et al. (+9 more)
ScAu$_2$Al is the Heusler
superconductor with the highest reported transition temperature, Tc, close to 5 K. Here, we combine high-resolution ac calorimetry and electrical-transport measurements with first-principles-based Eliashberg calculations. The large specific-heat jump, ${\Delta}C/{\gamma_nT_c} =$ 2.2, well above the weak-coupling BCS value of 1.43, together with an electron-
phonon coupling constant ${\lambda} = $1.1, establishes ScAu2Al as a strong-coupling
superconductor. The electronic specific heat is described better by a two-gap {\alpha} model, with $2{\Delta}_S/k_BT_c = $ 4.1 and $2{\Delta}_L/k_BT_c = $ 4.8, than by a single-gap model. The thermodynamic upper critical field follows a conventional WHH-like temperature dependence with $B_{c2}(0) = 0.18$ T, whereas the resistively determined critical field reaches values about three times larger and exhibits a pronounced positive
curvature. We attribute the enhanced resistive field scale to superconductivity in disordered grain-boundary and interfacial regions. The experimental results are supported by calculations of the heat capacity and upper critical field within the Eliashberg formalism using Fermi-surface and electron-
phonon parameters obtained from first-principles calculations
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All authors Jozef Kačmarčík, Zuzana Pribulová, Yevhen Petrenko, Gabriel Kuderowicz, Paweł Wójcik, Michał J. Winiarski, Szymon M. Królak, Filip Košuth, Pavol Szabó, Tomasz Klimczuk, Bartlomiej Wiendlocha, Peter Samuely
Superconductivity
Failure of Conventional Roughness Metrics in Assessing Field-Limiting Mesoscopic Topography in SRF Nb Films on Cu
Eric M. Lechner, Brandi Redman, Theodore S. Cahall et al. (+1 more)
A characteristic corrugated surface morphology of Nb films on Cu is identified, with mesoscopic features comparable to the London penetration depth and coherence length, and its impact on superconducting
radio frequency cavity performance metrics is examined.
Magnetic field enhancement factors and superheating field suppression factors are calculated within the London model for a representative corrugated geometry. These results demonstrate that roughness trends derived from buffered chemical polished and electropolished Nb cavities do not capture the impact of nanoscale surface morphology on SRF Nb thin film performance, as even surfaces that possess low average roughness can contain
geometric features that generate substantial local field enhancement and significantly suppress the Bean Livingston barrier. The influence of surface roughness on impurity
diffusion is also investigated, which highlights the roles of increased surface area and local
geometric confinement in modifying near surface impurity distributions. Tracking the effect of impurity
diffusion on the
evolution of
magnetic field enhancement, we show that geometrically confined impurity distributions can mitigate nanoscale
magnetic field enhancement substantially.
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All authors Eric M. Lechner, Brandi Redman, Theodore S. Cahall, Anne-Marie Valente-Feliciano
Statistical Mechanics
Non-negative differential mobility of one-dimensional underdamped Langevin dynamics: Proof via mapping to a marginally stable oscillator
Udo Seifert
For an underdamped particle driven by a constant non-conservative force across a periodic potential in one spatial dimension at finite temperature, numerical studies have shown that the differential mobility is non-negative. Here, an analytical proof will be given that is based on expressing the differential mobility by the long-time mean position of a marginally stable harmonic oscillator subject to noise in its stiffness.
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Superconductivity
Topological properties and gap structure of the paired state in twisted TMD bilayers within a two-band effective model
Palash Saha and Michał Zegrodnik
We present a theoretical study motivated by the recent experimental results which demonstrate superconductivity emerging from flat topological bands of twisted transition metal dichalcogenide (TMD) bilayers. To capture the non-trivial band
topology of the system, we employ an effective two-band Kane-Mele-like model and substitute it with Coulomb repulsion and inter-site pairing interactions treated at the Hartree-Fock mean-field level. Assuming a real-space pairing scenario, we analyze the resulting superconducting gap
symmetry and the stability of the paired state as a function of band filling and applied displacement field. Finally, we highlight the interplay between superconductivity and non-trivial
topology, detailing how interaction-induced effects and the
evolution of the density of states shape the resulting superconducting phase diagram.
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Materials Science
Multivalley 3D Electronic Structure of PbSe from Soft-X-Ray ARPES and First-Principles Calculations
Zefeng Cai, Valentine V. Volobuev, Jędrzej Korczak et al. (+7 more)
PbSe is a narrow-gap IV-VI
semiconductor, whose multivalley valence bands, with maxima at the L, $\Sigma$, and $\Delta$ points, underpin its intermediate-temperature thermoelectric properties. We combine soft-X-ray angle-resolved photoemission spectroscopy (SX-ARPES) with first principles simulations to study the valence band structure of bulk PbSe. High resolution measurements are conducted at
photon energies of 400-900 eV to map the valence
manifold along X$\Gamma$X, WXW, and K$\Gamma$K, and iso-energy surfaces are collected in the $k_z=0$ plane. Comparison to ARPES enables a rigorous assessment of the performance of density functional theory (DFT), using semi-local and hybrid functionals, as well as many-body perturbation theory within the quasiparticle self-consistent $GW$ approximation. We find that the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional and QP$GW$ reproduce the measured band dispersions to within 0.1-0.2 eV over the entire valence band. In contrast, the semi-local Perdew-Burke-Ernzerhof (PBE) functional compresses the band width and deviates from experiment by up to 0.6 eV. We further show that an accurate band structure and band gap are vital to obtaining a correct description of the dependence of the Seebeck coefficient of p-type PbSe on the hole concentration (Pisarenko relation). This has implications for computational efforts to discover thermoelectric materials.
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All authors Zefeng Cai, Valentine V. Volobuev, Jędrzej Korczak, Enrico Della Valle, Hantian Liu, Moritz Hoesch, Sergey M. Frolov, Tomasz Story, Vladimir N. Strocov, Noa Marom
Materials Science
Electrodeposited Co and Ni Hexacyanoferrates: Insights into Structure and Morphology
Larissa de O. Garcia, Michael Pohlitz, Mohammed F. Kalady et al. (+3 more)
Prussian blue (PB) and its analogues (PBAs) are interesting materials for electrochemical applications due to their tunable redox chemistry and open framework structure. In this study, hexacyanoferrates (HCF) containing iron (FeHCF), cobalt (CoHCF) and nickel (NiHCF) were synthesized via potentiostatic electrodeposition. Cyclic voltammetry revealed distinct redox behaviors. Morphological characterization (SEM, EDX) demonstrated uniform, pyramidal film growth for FeHCF and CoHCF. Otherwise, NiHCF presented a cracked film with cubic clusters on top due to residual stress. Despite this, homogeneous element distribution was found for all samples. Structural characterization (TEM and XRD) confirmed a cubic
lattice crystal structure for all films, with systematic
lattice contraction from Fe to Co to Ni due to decreasing atomic radius. Raman and XPS data revealed a shift toward Fe2+ dominant oxidation states and modifications in CN bonding with the influence of K+ and water occupancy in the PBAs framework. These findings illustrate how metal substitution and deposition parameters can tune the structural and electrochemical properties of PBA films, presenting a strategic route to design tailored electrodes.
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All authors Larissa de O. Garcia, Michael Pohlitz, Mohammed F. Kalady, Falk Röder, Axel Lubk, Daniel Wolfmand Christian K. Müller
Materials Science
Complete Neural Electronic Initialization Accelerates Materials DFT
Felix Ærtebjerg, Jonas Elsborg and Arghya Bhowmik
We present the first complete machine learning method for accelerating plane-wave density functional theory (DFT) in materials under the projector augmented wave (PAW) formalism. We formalize seven criteria that a Complete Neural Electronic Initializer must satisfy for practical end-to-end PAW DFT acceleration. Applying these to prior work reveals two structure-dependent components, augmentation occupancies and
spin initialization, whose absence prevents existing acceleration methods from providing complete reference-free initialization. We show that omitting these components can eliminate or reverse the acceleration obtained via models that only predict the smooth valence density. We satisfy the missing requirements by introducing AugNet, a general equivariant model for PAW augmentation occupancies, and the first general
spin density model for materials, which predicts the smooth
spin-difference density and
spin-difference PAW augmentation occupancies using predicted
magnetic moments to constrain the global
magnetic state. Combined with existing valence density models, our full method satisfies all seven criteria and forms a fully reference-free electronic initializer for materials DFT, requiring no electronic quantities from a converged target calculation. We show that perfect initialization could cut PAW DFT wall time by 40-52%, and our method recovers up to 62% of this saving, reducing end-to-end DFT wall time by up to ~25% on unseen structures while preserving converged energies.
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Statistical Mechanics
Full macroscopic thermalization and the formation of Schrödinger's cats by unitary time evolution in the weakly perturbed Ising model - Applications of the Roos-Sugimoto-Teufel-Tumulka-Vogel theory
Hal Tasaki
We study macroscopic thermalization in the ferromagnetic Ising model with a weak generic (highly nonlocal and many-body) random
quantum perturbation. We prove that a single typical perturbation makes every initial state $|\Phi(0)\rangle$ in a specified energy shell thermalize: a measurement of a large class of macroscopic observables in the time-evolved state $|\Phi(t)\rangle=e^{-i\widetilde H_Lt}|\Phi(0)\rangle$ yields their thermal
equilibrium values, within a prescribed precision and with overwhelming probability, at sufficiently large, typical times $t$. The result covers the full finite-temperature range, including the ordered phase, where we use plus boundary conditions. With periodic boundary conditions in the ordered phase, every energy eigenstate in the shell is shown to be an almost balanced macroscopic Schrödinger-cat state. Moreover, every pure initial state becomes such a cat at sufficiently large, typical times, with each branch in the corresponding macroscopic thermal
equilibrium. The proof combines the theory of Roos, Sugimoto, Teufel, Tumulka, and Vogel with rigorous Ising large-deviation estimates.
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Mesoscale and Nanoscale Physics
Daniel Erkensten, Katarzyna Sadecka, Marie-Christin Heißenbüttel et al. (+2 more)
Two-dimensional
magnetic semiconductors provide a unique platform for exploring the interplay between charge,
lattice,
spin, and light. The strong Coulomb interaction combined with long-range
magnetic order enables efficient coupling between
excitons and both
lattice vibrations and
spin excitations, thereby strongly influencing
exciton dynamics and transport in these materials. Here, we develop a microscopic many-body theory of
exciton–
phonon and
exciton–
magnon interactions in mono- and bilayer CrSBr. We demonstrate that the quasi-one-dimensional character of
excitons in CrSBr gives rise to strongly anisotropic
exciton–
phonon and
exciton–
magnon scattering, resulting in direction-dependent
diffusion coefficients. Furthermore, we identify a distinct temperature dependence originating from one-
phonon and two-
magnon scattering processes, providing characteristic signatures of the underlying microscopic interactions. Our work establishes a microscopic framework for understanding
exciton transport in
magnetic van der Waals
semiconductors and provides guidance for future optical spectroscopy and
exciton transport experiments.
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All authors Daniel Erkensten, Katarzyna Sadecka, Marie-Christin Heißenbüttel, Thorsten Deilmann, Ermin Malic
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Floquet physics from quantized light-matter interaction: geometric phases, gauge consistency, and entanglement
Beatriz Pérez-González, Sigmund Kohler and Mónica Benito
Floquet engineering and cavity
quantum electrodynamics represent two complementary descriptions of light-matter interaction, yet their precise connection beyond the small-coupling regime remains subtle. Here, by representing the cavity field in a large-
photon-number phase basis, we show that the time-dependent Schrödinger equation of a classically driven system emerges directly from the fully quantized problem, without replacing the field by a classical trajectory or assuming a mean-field decoupling. This establishes a one-to-one correspondence between quasienergies and
photon-sector-shifted
quantum energies, while providing a direct
quantum interpretation of the Floquet mean energy and Anandan phase. Because gauge invariance is exactly preserved at the level of the
quantum Hamiltonian, the
quantum-to-classical correspondence remains valid beyond perturbative and small-coupling regimes. We further show that light–matter
entanglement is encoded in matter–harmonic correlations of the corresponding Floquet mode, thus identifying a semiclassical limit in which Floquet physics emerges without requiring the
quantum field itself to approach a coherent classical state.
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Superconductivity
Universal Dzyaloshinski-Moriya interaction dictates pairing in unconventional superconductor families
Baishun Yang, Yida Chu, Xuelei Sui et al. (+3 more)
The collinear-antiferromagnetic
spin-fluctuation paradigm has long guided unconventional superconductivity research, yet fails to reconcile the noncollinear
spin phenomena observed across cuprates, iron-based
superconductors, and nickelates. Using extensive first-principles calculations and unbiased large-scale DMRG simulations, we show that Dzyaloshinski-Moriya interaction (DMI)-arising from local inversion-
symmetry breaking-is a common ingredient across these families. This DMI unifies hallmark observations in parent compounds-incommensurate orders,
spin-wave gaps, and noncollinear textures. Under hole doping, strong DMI drives
spin vortices to merge with pi-shifted hole stripes, forming hybrid
vortex-hole stripe phases. These phases stabilize charge order while supporting, not suppressing, superconductivity. By contrast, under electron doping, these
vortices pin holes and suppress long-range superconductivity. Our results establish DMI as a unifying link between noncollinear magnetism and superconductivity, identifying hole-strip-
vortex coupling as a microscopic pairing engine. Given that DMI is common across major
superconductor families, these findings challenge the prevailing pairing mechanism and offer an experimentally testable roadmap for materials optimization.
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All authors Baishun Yang, Yida Chu, Xuelei Sui, Haiqing Lin, Shijie Hu, Bing Huang
Other Condensed Matter · Cross-listed from physics.optics
Symmetry Properties of Bound States in the Continuum in Hexagonal Lattice Photonic Crystal Slabs
Rhys Jones and Sang Soon Oh
Bound states in the continuum (BICs) within photonic
crystal (PhC) cavities realise the full confinement of light despite lying within the continuum of radiation modes. While their diverse practical applications has attracted considerable research interest, developing the fundamental understanding of BICs is important for the design of future devices. This paper uses a group theoretical approach to determine the basis functions of the irreducible representations (irreps) of a PhC with the
symmetry of the $C_{6v}$ space group, from which the spatial distribution of the electromagnetic fields within the cavity can be determined. Furthermore, the reflection parity of the electric field and a $k \cdot p$ perturbation theory approach are used to determine the far-field polarisation of each irrep. The $k \cdot p$ perturbation explains why the $E_2$ irrep has a topological charge of $-2$, as well as showing that a topological charge does not necessarily indicate the presence of a BIC. Additionally, it is found that the quality factor of the modes do not necessarily scale as $Q \propto 1/|k|^{2|q|}$, but are instead dependent on the coupling to the radiative mode in the $k \cdot p$ perturbation, which is determined from the
symmetry of each irrep.
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Superconductivity
Collective excitation-mediated transport in nanoscale Josephson junctions that exhibit quantum confinement
Zhengyuan Liu, Sebastian Scherb, Werner M.J. van Weerdenburg et al. (+3 more)
Quantum confinement can strongly modify transport through Josephson junctions. Here, we study local tunneling transport through nanoscale Josephson junction stacks in the Coulomb blockade regime, where the metallic layers exhibit strong vertical
quantum confinement. We find that quasiparticle transport is strongly enhanced by a collective excitation mode intrinsic to the junction and localized in the isolated metallic overlayer. We quantify both the collective-mode energy and the Coulomb gap and show that both exhibit strong layer-dependent modulation, consistent with the modulation of the underlying
quantum well states. We further investigate how the collective excitation responds to various perturbations, including mechanical motion and an applied
magnetic field. Our results suggest that this collective mode is sensitive to quasiparticles near the Fermi level and may therefore provide an indirect probe of the superconducting state.
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All authors Zhengyuan Liu, Sebastian Scherb, Werner M.J. van Weerdenburg, Daniel Wegner, Nadine Hauptmann, Alexander A. Khajetoorians
Statistical Mechanics
Self-avoiding trails in two and three dimensions
Xiaodi Su, Zongzheng Zhou and Qianqian Wu
The self-avoiding trail is an important variant of the self-avoiding walk. In this work, we employ an irreversible Markov chain Monte Carlo
algorithm, together with the reversible Berretti-Sokal
algorithm for comparison, to simulate self-avoiding trails on the square and simple cubic
lattices with periodic boundary conditions. Based on finite-size analyses of the unwrapped end-to-end distance and the Binder ratio, we accurately estimate the critical points on the simple cubic and square
lattices to be 0.206\,376\,9(2) and 0.367\,561\,1(1), respectively, improving the precision of previous best estimates by factors of 500 and 70. At the estimated critical point of the simple cubic
lattice, we numerically demonstrate that both the critical scaling behaviors of various quantities and the length distributions of self-avoiding trails and walks are consistent with each other. Our accurate numerical results are attributed to the efficiency of the irreversible
algorithm, whose advantage over the reversible
algorithm is even more pronounced for the self-avoiding trail model than for the self-avoiding walk model.
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Materials Science
Adsorption of Phosgene Gas on Pristine and Noble Metal-Doped B12N12 Nanocages: Insights from Density Functional Theory
Shahariar Chowdhury, Mohammad Abdul Matin, Samiran Bhattacharjee et al. (+1 more)
This study examines phosgene (COCl2) adsorption on pristine and noble metal-doped (Ag, Au, Pd, Pt) B12N12 nanocages using dispersion-corrected density functional theory [B3LYP-D3(BJ)]. Boron-site substitution narrows the HOMO-LUMO gap far more than nitrogen-site substitution (69-82% vs 41-67%) and was adopted throughout. All systems were fully optimized; multiple starting geometries converged to two stable minima per dopant, X-O and X-Cl. Pristine B12N12 binds phosgene weakly (Eads = -12.8 to -24.9 kJ mol^-1). Doping strengthens binding, spanning -14.0 to -60.7 kJ mol^-1 after counterpoise correction, with platinum exhibiting the highest affinity and structural stability. Natural population analysis indicates phosgene donates at most 0.33 e, and QTAIM classifies all cage-adsorbate bond critical points as closed-shell or intermediate, confirming physisorption. Vibrational frequency analysis confirms all structures as true minima and reveals that the adsorption
entropy penalty (59-161 J mol^-1 K^-1) is decisive. Under Grimme's quasi-harmonic approximation, pristine cages fail to bind phosgene (Delta G = +23.8 to +27.9 kJ mol^-1), whereas only Pt-O (Delta G = -8.8 kJ mol^-1) and Pd-O (-7.2 kJ mol^-1) adsorb phosgene spontaneously at 298 K. Transition-state theory indicates rapid room-temperature recovery (tau = 43 ms for Pt-O), while thermodynamic desorption occurs at 69.6 deg C, ensuring a practical regeneration window. Silver-doped cages provide narrow post-adsorption gaps and high electrophilicity but cannot retain phosgene at ambient conditions. Platinum doping is therefore the most effective strategy for reversible phosgene detection using B12N12 nanocages.
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All authors Shahariar Chowdhury, Mohammad Abdul Matin, Samiran Bhattacharjee, Ishtiaque M. Syed
Mesoscale and Nanoscale Physics
Field-free and Sub-ns Magnetization Reversal in Synthetic Anti-ferromagnets using Sequential Spin-Orbit Torque and Spin-Transfer Torque Pulses
Subham Das and Naveen Sisodia
Single free-layer
Magnetic Tunnel Junctions (MTJs) with a perpendicular easy axis driven by
spin currents have demonstrated fast, deterministic switching, non-
volatility, and low-power
memory capabilities when integrated with modern nanoelectronic devices. In this article, we numerically investigate sub-ns and field-free switching characteristics of a p-MTJ with a Synthetic Antiferromagnet (SAF) based free layer structure using sequential sub-ns current pulses of
Spin Orbit Torque (SOT) and
Spin Transfer Torque (STT). While the SOT pulse applied to one layer decreases the transition time by bringing the coupled layers into an in-plane configuration, the subsequent STT pulse on the system determines the switching direction. Various pulse protocols have been investigated for both SOT and STT current amplitude and pulse widths, while also considering the effect of the Reference Layer (RL). While significant regions of deterministic switching events are observed, limitations in SOT, STT parameters are further investigated in presence of thermal fluctuations.
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Strongly Correlated Electrons
Orbital character and photoemission signatures of the A-pocket in infinite-layer nickelates
Leonard M. Verhoff, Michael Seidl, Liang Si et al. (+1 more)
The orbital character of the so-called $A$-pocket in infinite-layer nickelates remains under debate. We investigate bulk NdNiO$_2$ using density-functional theory combined with dynamical mean-field theory and identify the $A$-pocket as a strongly hybridized state with substantial interstitial-$s$, Nd-$d_{xy}$ and Ni-d_${xz/yz}$ contributions. Our angle-resolved photoemission spectroscopy simulations reproduce the main polarization-dependent features measured experimentally. Removing the interstitial-$s$
matrix element leads to a clear disagreement with the measured Fermi surface maps, even though the interstitial-$s$ contribution is not dominant. Suppressing instead the Nd-d$_{xy}$ or Ni-$_{dxz/yz}$
matrix elements leaves the maps almost unchanged, because these are small at the zone corner. More generally, our results demonstrate that assigning the microscopic origin of spectral features in infinite-layer nickelates requires treating orbital hybridizations and photoemission
matrix elements together.
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All authors Leonard M. Verhoff, Michael Seidl, Liang Si, Karsten Held
Materials Science
Material chemical composition impacts on the band alignments: preliminary results on the 3D/2D perovskite interfaces
Philippe Baranek
While hybrid organic-inorganic halide perovskite solar
cells have achieved remarkable certified efficiencies, their widespread industrial and societal adoption is hindered by instabilities against light, heat, and moisture. A strategy to mitigate issues, such as moisture sensitivity, involves depositing 2D perovskites on 3D thin films for passivation, but their precise impact on
cell performance requires detailed understanding. This work employs atomistic first-principles simulation, based on hybrid functionals, to investigate the structural and electronic properties of 2D/3D interfaces. Specifically, it focuses on interfaces formed by (PEA)2PbI4 (PEA = C6H5CH2CH2NH3, as 2D perovskite) and complex 3D perovskites (Cs0.125MA0.14FA0.735Pb(I0.87Br0.13)3, with MA = CH3NH3 and FA = CH(NH2)2) which are ones of the most used perovskites. The approach is based on optimized hybrid functional methods to consistently determine work function (WF), electron affinity, and band offset (EBO) across different layers. Preliminary results demonstrate that WF are strongly dependent on the cleavage and chemical nature of the surfaces. Furthermore, the creation of interfaces significantly impacts macroscopic potential, with a noticeable slope linked to the polarization of the 3D materials due to ionic substitutions. The band
alignment is also highly sensitive to the termination and chemical nature of the perovskites, with valence and conduction EBOs varying from 0.2 to 0.7 eV and of approximately 0.25 eV, respectively. This ongoing work aims to provide a systematic description of these effects, with further investigation into the impact of dimensionality loss on perovskite stability.
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Strongly Correlated Electrons
Reformulating Pfaffian Quantum Monte Carlo with the Hybrid Monte Carlo formalism
Thomas Hauschild, Lin Wang, Finn L. Temmen et al. (+1 more)
Pfaffian
quantum Monte Carlo extends auxiliary field methods to anomalous fermionic
Hamiltonians, opening access to pairing systems and Hubbard–Stratonovich decompositions that are unavailable in determinant formulations. We show that this flexibility can be combined with the global updates of Hybrid Monte Carlo (HMC), leading to an
algorithm scaling as $\mathcal{O}(N_\tau L^3)$, similar to its determinant
quantum Monte Carlo (DQMC) counterpart without pseudofermions. The central observation is that the molecular dynamics force depends only on the magnitude of the Pfaffian weight, which admits a determinant representation and leads to a simplified force expression. This construction also yields unequal-time Majorana correlators at essentially no additional cost. We demonstrate the method for the interacting Kitaev chain using the imaginary and symmetric Majorana hopping channel. The resulting noncommuting auxiliary-field bond operators are treated through an even-odd decomposition. Their block structure permits an analytic reduction to a sparse effective interaction
matrix of bandwidth two and allows explicit force expressions. Results agree with exact diagonalization and resolve low-energy modes localized at opposite boundaries, together with their increasing overlap near the topological transition of the non-interacting system. These findings establish HMC as an efficient sampling framework for Pfaffian auxiliary-field simulations.
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All authors Thomas Hauschild, Lin Wang, Finn L. Temmen, Thomas Luu
Mesoscale and Nanoscale Physics
Spatially resolved quantum magnetometry and stray-field reconstruction of permalloy microdisks using boron-vacancy centers in hexagonal boron nitride
Peiting Wen, Shuyu Wen, Jiang Qu et al. (+13 more)
Transferable hexagonal boron nitride (hBN) hosting negatively charged boron-vacancy (VB$^{-}$)
spin defects offers a versatile platform for integrated
quantum magnetometry, yet quantitative imaging of
magnetic microstructures remains challenging. Here, we integrate a transferred hBN flake with a 4 $\mu$m-diameter permalloy (Py = Ni${81}$Fe${19}$) microdisk and perform spatially resolved optically detected
magnetic resonance measurements at room temperature. An applied in-plane
magnetic field distorts the
vortex-state magnetization, generating edge-localized
magnetic surface charges and pronounced stray-field signatures at opposite disk edges. By referencing each pixel to its local zero-field splitting and correcting for a residual out-of-plane bias field, we quantitatively reconstruct the out-of-plane stray-field distribution, revealing peak fields of approximately 11.2 mT. An edge-charge model reproduces the spatial distribution and amplitude of the reconstructed field, linking the ODMR response to the field-driven
evolution of the
vortex state. These results establish transferred hBN VB$^{-}$ sensors for quantitative magnetometry of
magnetic microstructures.
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All authors Peiting Wen, Shuyu Wen, Jiang Qu, Zeling Xiong, Katrin Schultheiss, Slawomir Prucnal, Artur Erbe, Kenji Watanabe, Takashi Taniguchi, Jürgen Lindner, Jürgen Fassbender, Manfred Helm, Shengqiang Zhou, Ruslan Salikhov, Helmut Schultheiss, Yonder Berencén
Other Condensed Matter · Cross-listed from quant-ph
Semiclassical Arrhenius law for quantum-thermal escape rates
Luca Salasnich and Cesare Vianello
The
quantum-thermal escape rate from a metastable well in the absence of dissipation is customarily written as a Boltzmann average of the Hill-Wheeler flux over a continuum of energies. However, this continuum treatment diverges exponentially at low temperature. The divergence originates in a mismatch between a
quantum partition function and a classical flux integral; retaining the discrete nature of the quasibound spectrum removes it exactly, and the resulting semiclassical Arrhenius law is obtained in closed form on both sides of the crossover temperature. Evaluating a uniform Kemble transmission probability on Bohr-Sommerfeld levels further recovers the standard WKB decay rate of the lowest resonance at zero temperature. Benchmarked against the resonances of cubic and quintic potentials, obtained by complex scaling, this uniform semiclassical result stays within $9\%$ of the exact rate over eleven orders of magnitude.
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Strongly Correlated Electrons
Resonant spin mode from retarded relaxation
J. Beater and G.S. Uhrig
In ultrafast
spin dynamics additional modes in the THz range appear which cannot be captured by conventional Landau-Lifshitz equations. Extending these equations by further time derivatives (inertial Landau-Lifshitz-Gilbert equation) solves the issue only partly. Here we suggest to explain these modes by retarded Lindbladian relaxation. In spite of the simplicity of this phenomenological approach the agreement with the experimental data is promising: physically plausible parameters describe the position and the large width of these resonances. The salient feature of several almost equidistant resonance peaks is in-line with experimental evidence as well.
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Superconductivity
Digital-analog concept for superconducting perceptron-like neural networks
Andrey E. Schegolev, Vsevolod I. Ruzhickiy, Georgy I. Gubochkin et al. (+7 more)
A promising route to superconducting artificial neural networks is a hybrid digital-Analog architecture that combines digital single-flux-
quantum (SFQ) communication with compact Analog nonlinear processing. The study focused on the dynamic conversion of a discrete signal passing through a digital-to-Analog-to-digital (DAD) converter, in which the role of the Analog
cell was performed by a $\Sigma$-
neuron with a nonlinear transfer function – the basic
cell of perceptron-like neural networks. Furthermore, the DAD converter, the elementary functional block of the hybrid architecture, combines a digital-to-analog converter (DAC) and an Analog-to-digital converter (ADC), and re-encodes the Analog $\Sigma$-
neuron waveforms as an SFQ pulse sequence.
Circuit-level simulations demonstrate how input values encoded by SFQ pulse trains are converted into analog signal levels, transformed by the $\Sigma$-
neuron, and mapped back to pulse-based outputs. As a key experimental step, we fabricated and characterised a redesigned $\Sigma$-
neuron and measured a sigmoid-like transfer characteristic suitable for activation-function implementation. The extracted response was incorporated into system-level simulations to assess the influence of realistic device parameters on the conversion process. We delineate the operating-range matching requirements for the DAC,
neuron, and ADC blocks, supporting the feasibility of the proposed interface as a building block for perceptron-like superconducting neural networks with digital inputs and outputs. Finally, we developed two perceptron networks, one using a mathematical sigmoid activation and the other the measured $\Sigma$-
neuron transfer characteristic, which reached classification accuracies of $97.0\%$ and $91.9\%$, respectively, on the MNIST handwritten digit
dataset.
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All authors Andrey E. Schegolev, Vsevolod I. Ruzhickiy, Georgy I. Gubochkin, Alexander S. Ionin, Ivan A. Nazhestkin, Mikhail Y. Fominskii, Lyudmila V. Filippenko, Igor I. Soloviev, Maxim V. Tereshonok, Nikolay V. Klenov